This application relates to a method and apparatus for the production of polycarbonates via an interfacial process in which the NaCl brine produced at the end of the process is treated and recycled for use as a feedstock in the electrolytic production of chlorine.
The interfacial process for production of polycarbonates is well known, and is described inter alia in U.S. Pat. Nos. 3,945,969; 3,989,672 and 4,115,358. In general, the process as shown in FIG. 1 involves electrolytic production of chlorine from a concentrated (e.g. 20-30wt %) NaCl brine solution. The chlorine produced is combined with carbon monoxide to produce phosgene, which is reacted with a dihydric phenol such as bisphenol A in an interfacial polycarbonate facility. The interfacial polycarbonate facility produces polycarbonate, which is the desired product, and a waste brine which contains approximately 15wt % NaCl and various organic and inorganic impurities.
At present, this waste brine is simply disposed of as an industrial effluent. Doing such disposal properly is expensive, and may nevertheless place a burden on the environment. It would therefore be desirable to be able to recycle this brine in a useful manner consistent with the needs of the interfacial polycarbonate plant.
Recycle of NaCl brines from various sources is known in the art. For example, it is common practice to treat the anolyte brine which is produced as a waste product from the electrolytic production of chlorine to produce a more concentrated brine which can be utilized once again as a feedstock for electrolytic production of chlorine. Various processes for treating brine for use in electrolysis reactions are disclosed in U.S. Pat. Nos. 3,970,528, 4,060,465, 4,078,978, 4,115,219, 4,119,508, 4,132,759, 4,415,460, 4,470,891, 4,746,441, 4,747,917, 4,855,059, 5,034,129, 5,082,302, 5,126,019, 5,352,362 and 5,681,447. These disclosures all make it clear that membranes used in electrolytic processes are very sensitive to numerous types of impurities, and therefore that careful purification of any recycled brine is required to avoid fouling the electrolytic cell. Because of this sensitivity, any consideration of utilizing recycled brines derived from processes other than the electrolysis process itself which may contain additional impurities must address these additional impurities and provide for their removal as necessary.
U.S. Pat. Nos. 4,126,526 and 4,415,460 disclose processes for removal of organic materials from a recycle brine prior to reintroduction of the brine to a chlorine-producing electrolysis cell. In the first case, chlorine is used to convert the organic impurities to more volatile organic compounds which are separated from the brine by stripping. This process has the advantage of using materials already on hand (i.e., chlorine), but has the disadvantage of producing a waste stream containing chlorinated hydrocarbons which may themselves be undesirable to introduce into the environment. In the second case, organic impurities arising during the production of olefin oxides are oxidized with chlorate ions at a pH of 5.0 or less to convert the organic impurities into carbon dioxide. The differences in the nature of the organic impurities found in waste streams from olefin oxide production and those from interfacial polycarbonate production, and the reduced oxidizing power of chlorates make it doubtful, however, that the same process would be effective to produce a usable recycled brine in the latter process. Furthermore, utilization of chlorates for oxidation would result in yellowing of the product which is undesirable.
U.S. Pat. No. 5,505,856 discloses an ozonation process for the removal of organic materials from contaminated water. U.S. Pat. No. 5,178,755 discloses a combination of ozone and UV irradiation for treatment of contaminated water. (see also U.S. Pat. No. 5,888,403). These processes are not used to remove organic contaminants from a brine stream for recycling, and are not used for the same types of contaminants which are found in the waste brine streams from the production of polycarbonates.
We have now identified a process for treating brine produced in an interfacial polycarbonate plant to render it useful for recycle to the electrolytic cell. In accordance with an embodiment of the invention, this process includes the steps of
(a) combining the waste brine stream with ozone to produce an ozonated brine in which the concentration of at least some of the organic impurities is reduced;
(b) acidifying the ozonated brine to remove carbonates and produce a decarbonated brine; and
(c) concentrating the decarbonated brine to produce a treated brine in which the concentration of NaCl is at least 10 wt % NaCl, and preferably at least 20%. A filtration step may be included prior to the ozonation to remove particulate impurities.
When used in an interfacial polycarbonate plant in combination with chlorine, phosgene and polycarbonate producing facilities, the method of the invention provides an environmentally superior and highly cost effective method for the producing polycarbonate.